Analysis of ultrasonic vocalisation does not allow chronic pain to be evaluated in rats

Most pain tests used for the assessment of drug analgesic activity in animal chronic pain models are based on the measurement of the response to an external acute stimulation (thermal, mechanical or electrical). But these stimuli are not related to the chronic pain experienced by the animal. Quantitative analysis of the spontaneous behaviour induced by the chronic pain state is needed. Several authors have suggested that ultrasonic vocalisations (USVs) emitted by rats in painful situations might reflect expression of affective pain. In a first study, we recorded spontaneous USVs in sub-chronic and chronic pain models: inflammation induced by carrageenan, arthritis induced by Freund's adjuvant and diabetes induced by streptozotocin. The USVs were analysed when naive Sprague-Dawley rats were alone and during non-agonistic interaction with a conspecific. When the rats were alone they did not emit any USV. During social interaction, no difference in either the frequency or the duration was observed between the emissions of healthy rats and rats in pain. In a third study, the influence of three parameters, degree of confrontation between the rats, age of the conspecific and housing conditions (isolated or collective) was studied in the arthritic rat model. Arthritic rats did not emit more USVs than controls in any of our experimental conditions. A fourth study showed that Aspirin (200 mg/kg) had no effect on the USVs, this data confirms the lack of direct relationship between USVs and experimental chronic pain in rats in our conditions.